The present invention relates to a security paper with authenticity features in the form of coloring agents with magnetic properties added thereto or applied thereon.
It is already known to print security papers i.e. bank notes, pass cards, documents and the like with magnetic inks or to embed magnetizable material in them. Security papers prepared in this way can, following corresponding magnetic excitation, be surveyed by automatic machinery and checked for their authenticity. As suitable magnetizable materials preferably oxidic iron compounds such as .gamma.-ferric oxide or ferrosoferric oxide or also ferrites are used. These compounds have in each case a particular color, wherein the material becomes darker the stronger its magnetic properties are (German Patent Specification No. 843 660).
Although providing security papers with magnetic inks and/or authenticity features constitutes a substantial step forward in the direction of automatically acceptable bank notes or an automatically acceptable security paper, a check or evaluation of the magnetic properties of the security paper is insufficient by present day security standards. In recent times it has become more and more a question of testing the bank notes with non-visible electromagnetic radiation and thereby of undertaking particular checks. Since a series of printing inks used in bank note printing practically do not absorb in the IR-region of the optical spectrum, i.e. in this range appear colorless (IR-colourless) or "white" (IR-white), it has already been proposed to irradiate bank notes printed in this way with IR-light and to evaluate the proportion of the irradiation remitted. With this process it is possible to determine the condition of a bank note independently of the printed images over which the test trace passes. Since contamination in the IR-region of the spectrum shows the same optical properties as in the case of irradiation with visible light, in this way the degree of contamination can be determined uninfluenced by the particular printed image. This type of testing process requires security papers with prints on them from the printing inks already mentioned which are correspondingly translucent in the IR-region of the spectrum.
There have also already been tests to develop magnetic coatings which in the visible region of the spectrum are translucent so that corresponding use is not particularly noticed by the naked eye. A forger could in such cases overlook such coatings and accordingly make an attempt at forgery obvious (British Patent Specification No. 1 514 758). In the scientific world however the view is that the materials noted are ferromagnetic only at very low temperatures. Use of the materials in magnetic inks in security papers accordingly does not appear to be useful.
It is further known to use the semi-transparency of some magnetic materials in the IR-region of the spectrum in order to detect reflective markings lying below a magnetic data trace by corresponding irradiation. The application of information on to a data track however requires a magnetic material with a remanence of a particular degree. The known magnetic materials which have the required remanence indeed remain, applied in thin layers, somewhat transparent in the IR-region of the spectrum so that, for example, an IR-radiation reflecting element arranged underneath a data track can still be detected. However, this cannot be considered as IR colorlessness in the inventive sense, which lies substantially in the region of the IR-colorlessness of the paper carrier for IR-radiation and accordingly also permit measurements of contamination (German Offenlegungsschrift No. 26 23 708).
Further a magnetic card is known from Swiss Patent Specification No. 588 740 in which an optical property of the magnetic data trace can be used for authenticity testing. However, also in this case, known magnetic operating materials are used for writing data on the card. These materials do not have the optical properties which are a prerequisite for the solution of the problem according to the present invention, which in particular are not sufficiently translucent in the IR.
In summary, it was accordingly known for carrying out particular tests to print security papers with inks which absorbed little in the IR-region of the spectrum. These materials however did not have any magnetic properties. On the other hand it was known to render security documents secure with magnetic materials. These magnetic materials however absorb over the whole optical spectral region even into the infrared relatively strongly. The combination of magnetic and transmission properties accordingly runs into difficulties.